This invention relates to an engine system and more particularly to an engine system employing hydrogen enhanced operation.
Hydrogen addition can be used to reduce pollution (especially NOx) from vehicles with spark ignition engines using gasoline and other fuels. Hydrogen can be produced by onboard conversion of a fraction of gasoline or other primary fuel into hydrogen rich gas (H2+CO) by partial oxidation in compact onboard devices suitable for vehicle applications. (See for example, Rabinovich, Bromberg, and Cohn 1995 U.S. Pat. No. 5,437,250 which discussed a plasmatron fuel converter device the contents of which are incorporated herein by reference). The very lean operation (very low fuel-to-air ratio) made possible by combustion of the hydrogen along with the gasoline results in significantly lower nitrogen oxide (NOx) emissions. Hydrogen addition allows for leaner operation without misfire (an unacceptably large fraction of failed ignitions) than would otherwise be possible. The allowed reduction in fuel/air ratio without misfire increases with increasing hydrogen addition. NOx emissions decrease strongly with decreasing fuel/air ratio. Very lean operation also provides higher engine efficiency. However, the increase in net efficiency is reduced by the energy loss in the gasoline-to-hydrogen conversion process. The increase in net efficiency is relatively modest in gasoline engines using conventional compression ratios (9–10.5).
Hydrogen addition can also be used to allow operation with larger amounts of engine gas recirculation (EGR) without misfire than would otherwise be possible. Increased values of EGR reduce in-cylinder burnt gas temperatures and thereby decrease NOx emissions. Operation with heavy EGR at conventional stoichiometric fuel/air mixtures (where the fuel/air ratio is sufficiently high so that there is no excess air over that needed for complete combustion) can result in substantial decreases in NOx. At a stoichiometric fuel/air ratio, the highly effective 3-way exhaust aftertreatment catalyst can be used for substantial additional NOx reduction. However, in contrast to very lean operation, there is likely to be little, no, or even negative net efficiency gain at conventional compression ratios. The use of hydrogen addition to promote lean operation or heavy EGR has been experimentally studied in conventional compression ratio gasoline engines. (See, for example, M. Greve, et al., 1999 Global Power Train Congress, Oct. 5–7, 1999, Stuttgart, Germany and I. E. Kirwin, et al., SAE Technical Paper 1999-01-2927 (1999)).
The relatively modest efficiency benefits of hydrogen enhanced lean operation at conventional compression ratios can be substantially increased by use of high compression ratio. A conceptual evaluation of high compression ratio, hydrogen enhanced lean burn gasoline engine operation (compression ratio 11 to 16) has shown that high compression ratio operation could more than double the net efficiency increase with net efficiencies gains greater than 20% being possible under some circumstances. (See, for example, Bromberg, et al., Intl. J. of Hydrogen Energy 24, 341–350 the contents of which are incorporated herein by reference). However, high compression ratio gasoline engine systems using hydrogen enhanced lean burn operation have received little attention and use of high compression ratio with hydrogen enhanced EGR for higher efficiency operation does not appear to have even been considered. An important reason for this lack of attention is that in order for high compression ratio, hydrogen enhanced engine systems to be practical, a demanding set of requirements must be met.
An important issue for high compression ratio, hydrogen enhanced gasoline engine operation is the avoidance of knock. Knock, the uncontrolled autoignition of the air/fuel mixture by compression rather than by spark ignition, can damage the engine. Gasoline engines are generally operated at compression ratios of 10.5 or lower in order to avoid knock. A gasoline engine operated at high compression ratio will experience knock if some means of reducing the octane requirement of the engine is not utilized. Knock can, in principle, be avoided by hydrogen enabled very lean operation of the gasoline engine at all times. However, use of very lean operation at higher engine torque and power levels can result in undesirable cost and performance. For a given fuel/air ratio, the amount of hydrogen required to maintain engine operation without misfire will increase with the increasing engine load. If the fuel/air ratio is not increased as the engine power increases, thereby allowing the hydrogen/gasoline ratio needed to prevent misfire to decrease, the hydrogen generation requirements will go up substantially with the increased gasoline flow that is required. The increased hydrogen requirements can increase the size and cost for an onboard gasoline-to-hydrogen fuel converter. Moreover, a vehicle engine that is operated very lean at high power levels would undergo a large reduction in torque and peak horsepower, which are required for rapid acceleration, hill climbing or towing; the amount of fuel in the engine cylinders is lower at lower fuel/air ratios, resulting in lower torque and power. If the engine operates very lean at all times, it will most likely be necessary to compensate for lower torque and peak power by use of a turbocharger or supercharger. Use of such boosting devices can result in a significant cost increase, particularly if large amounts of boost are needed.
Hence, requirements for high compression ratio, hydrogen enhanced lean burn gasoline engines are substantially more demanding than is the case for conventional compression ratio hydrogen enhanced lean burn engines. In conventional compression ratio engines there is no knock problem at any fuel/air ratio from very lean values up to stoichiometric values. Thus, in conventional compression ratio engines, when needed, the fuel/air ratio can be increased to provide higher values of torque and power without producing knock.
Another need for high compression ratio hydrogen enhanced gasoline engines is to minimize NOx emissions for a given amount of hydrogen addition at various levels of power. It is desirable to reduce overall NOx emissions to a level such that the cost and complexity of a lean NOx aftertreatment catalyst can be avoided. High compression operation may increase NOx emissions.
Control systems for hydrogen assisted natural gas engines have been previously considered. Collier, et al. describe a system (U.S. Pat. No. 5,787,864) and a method (U.S. Pat. No. 5,666,923) that uses a variable mixture of natural gas and hydrogen, in an engine that operates at variable air/fuel ratios. Hydrogen addition is used to extend the degree of lean operation at low power while high power operation (for acceleration, merging, climbing) uses near stoichiometric mixtures. A set of sensors is used to monitor for misfire in the natural gas engine. Collier does not consider the effect of knock on the control scenario and sensors are not used to measure knock; natural gas has a higher octane rating than gasoline which reduces the impact of knock at high compression ratio. In addition, natural gas has an inherently leaner combustion limit than gasoline and hydrogen requirements for extending the lean limit can be lower than those for gasoline. Moreover, Collier does not consider ways to minimize NOx emissions for a given amount of hydrogen addition so as to suppress NOx to extremely low levels. Collier also fails to consider the use of EGR for torque and power control and does not take the effect of EGR knock limit into account.